1. Field of the Invention
The present invention relates generally to power amplifiers and more particularly to Microwave Monolithic Integrated Circuits that employ Field Effect Transistors in power amplifiers.
2. Background of the Invention
Power amplifiers are widely found in various telecommunications systems, where they are typically employed to amplify a RF signal prior to transmitting the signal from an antenna. The power amplifier includes one or more semiconductor devices that are used for amplifying the signal. Conventional systems typically provide an amplified RF signal that has an output voltage that is limited by the maximum voltage capability of the semiconductor devices. Therefore, to increase the power output of the power amplifier the output current is increased. The size of the cabling that connects the power amplifier to the antenna is directly related to the RF signal output current. Larger cabling for larger currents entails increased costs for the cabling, larger connectors, increased cable mast support, and increased volume requirements for running the cable. Therefore, it is desirable to minimize the output current of a power amplifier.
Other design goals in a monolithic microwave integrated circuit (MMIC) power amplifier design include minimizing the supply current and reducing the impedance mismatch between the power amplifier and other system devices. The supply current to a power amplifier is dependent on the power output, the power dissipated, and the supply voltage at which the power amplifier is operated. Increases in the required output power and the power dissipation of the amplifier cause commensurate increases in the DC supply current to the device, resulting in increased line widths for the interconnections between the semiconductor devices and changes to the substrate to accommodate the higher supply currents. The increased line widths decrease the power amplifier power density and lead to increased parasitic capacitance associated with the interconnecting lines. Therefore, it is desirable to minimize the supply current of the power amplifier.
Mismatching between the power amplifier and other system components results in decreased drain efficiency and narrower bandwidth. Typical power amplifiers exhibit about 3 ohms of output channel impedance, which must be combined at several times the output impedance. For example, if 4 FETs are combined, this results in 200 ohms for a 50 ohm combined match. Thus, the transformation ratio is 200/3=67:1. Therefore, it is desirable to reduce the mismatch between the power amplifier and other system components.